The Third Generation Partnership Project (3GPP) initiative “License Assisted Long Term Evolution (LTE)” (LA-LTE) aims to allow LTE equipment to operate in the unlicensed 5 Gigahertz (GHz) radio spectrum. The unlicensed 5 GHz spectrum is used as an extension to the licensed spectrum. Accordingly, devices connect in the licensed spectrum (Primary Cell (PCell)) and use Carrier Aggregation (CA) to benefit from additional transmission capacity in the unlicensed spectrum (Secondary Cell (SCell)). To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the PCell is simultaneously repeated in the SCell.
Regulatory requirements, however, may not permit transmitting in the unlicensed spectrum without prior channel sensing. Because the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so called Listen-Before-Talk (LBT) scheme is applied. Today, the unlicensed 5 GHz spectrum is used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.”
LTE Overview
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT) spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
As illustrated in FIG. 2, in the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds (ms), each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each OFDM symbol is approximately 71.4 microseconds (μs).
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled; that is, in each subframe, the base station transmits control information about to which terminal's data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3, or 4 OFDM symbols in each subframe and the number n=1, 2, 3, or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of, e.g., the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
From LTE Release 11 onwards, the above described resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For LTE Release 8 to Release 10, only Physical Downlink Control Channel (PDCCH) is available.
The reference symbols shown FIG. 3 are the Cell specific Reference Symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
In a cellular communications system, there is a need to measure the channel conditions in order to know what transmission parameters to use. These parameters include, e.g., modulation type, coding rate, transmission rank, and frequency allocation. This applies to uplink as well as downlink transmissions.
The scheduler that makes the decisions on the transmission parameters is typically located in the base station, which in LTE is referred to as the enhanced or evolved Node B (eNB). Hence, it can measure channel properties of the uplink directly using known reference signals that the terminals (User Equipment devices (UEs) in LTE terminology) transmit. These measurements then form a basis for the uplink scheduling decisions that the eNB makes, which are then sent to the UEs via a downlink control channel.
However, for the downlink, the eNB is unable to measure any channel parameters. Rather, the eNB must rely on information that the UEs can gather and subsequently send back to the eNB. This so-called Channel State Information (CSI) is obtained in the UEs by measuring on known reference symbols, CSI Reference Symbols (CSI-RSs), transmitted in the downlink.
The CSI-RSs are UE specifically configured by Radio Resource Control (RRC) signaling, with a certain configured periodicity, T={5,10,20,40,80} ms (i.e., every Tth subframe). There is a possibility to configure both Non-Zero Power (NZP) CSI-RSs and Zero Power (ZP) CSI-RSs where the ZP CSI-RS is simply an unused resource that can be matched to a NZP CSI-RS in an adjacent eNB. This will improve the Signal to Interference plus Noise Ratio (SINR) for the CSI-RS measurements in the adjacent cell. The ZP CSI-RS can also be used as CSI Interference Measurement (CSI-IM) resources as introduced in LTE Release 11 and explained below.
In LTE, the format of the CSI reports are specified in detail and may contain Channel Quality Information (CQI), Rank Indicator (RI), and Precoding Matrix Indicator (PMI) (see, for example, 3GPP Technical Specification (TS) 36.213 version 11.6.0). The reports can be wideband or applicable to subbands. The reports can be configured by a RRC message to be sent periodically or in an aperiodic manner or triggered by a control message from the eNB to a UE. The quality and reliability of the CSI are crucial for the eNB in order to make the best possible scheduling decisions for the upcoming downlink transmissions.
The LTE standard does not specify in detail how the UE should obtain and average these measurements from multiple time instants. For example, the UE may measure over a time frame unknown to the eNB and combine several measurements in a UE-proprietary way to create the CSI values that are reported, either periodically or triggered.
In the context of LTE, the available CSI-RSs are referred to as “CSI-RS resources.” In addition, there are also “CSI-IM resources.” The latter are defined from the same set of possible physical locations in the time/frequency grid as the CSI-RSs, but with ZP, hence ZP CSI-RS. In other words, they are “silent” CSI-RSs. When the eNB is transmitting the shared data channel, it avoids mapping data to those resource elements used for CSI-IM. The resource elements used for CSI-IM are intended to give a UE the possibility to measure the power of any interference from another transmitter than its serving node.
Each UE can be configured with one, three, or four different CSI processes. Each CSI process is associated with one CSI-RS and one CSI-IM where these CSI-RS resources have been configured to the UE by RRC signaling and are thus periodically transmitted/occurring with a periodicity of T and with a given subframe offset relative to the frame start. If only one CSI process is used, then it is common to let the CSI-IM reflect the interference from all other eNBs, i.e. the serving cell uses a ZP CSI-RS that overlaps with the CSI-IM, but in other adjacent eNBs there is no ZP CSI-RS on these resources. In this way, the UE will measure the interference from adjacent cells using the CSI-IM.
If additional CSI processes are configured to the UE, then there is possibility for the network to also configure a ZP CSI-RS in the adjacent eNB that overlaps with a CSI-IM for this CSI process for the UE in the serving eNB. In this way, the UE will also feedback accurate CSI for the case when this adjacent cell is not transmitting. Hence, coordinated scheduling between eNBs is enabled with the use of multiple CSI processes where one CSI process feeds back CSI for the full interference case and the other CSI process feeds back CSI for the case when a (strong interfering) adjacent cell is muted. As mentioned above, up to four CSI processes can be configured to the UE, thereby enabling feedback of four different transmission hypotheses.
PDCCH and EPDCCH
The PDCCH/EPDCCH is used to carry Downlink Control Information (DCI) such as scheduling decisions and power control commands. More specifically, the DCI includes:                Downlink scheduling assignments, including Physical Downlink Shared Channel (PDSCH) resource indication, transport format, hybrid Automatic Repeat Request (ARQ) information, and control information related to spatial multiplexing (if applicable). A downlink scheduling assignment also includes a command for power control of the Physical Uplink Control Channel (PUCCH) used for transmission of hybrid ARQ acknowledgements in response to downlink scheduling assignments.        Uplink scheduling grants, including Physical Uplink Shared Channel (PUSCH) resource indication, transport format, and hybrid ARQ-related information. An uplink scheduling grant also includes a command for power control of the PUSCH.        Power control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.        
One PDCCH/EPDCCH carries one DCI message with one of the formats above. As multiple terminals can be scheduled simultaneously, on both downlink and uplink, there must be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on separate PDCCH/EPDCCH resources, and consequently there are typically multiple simultaneous PDCCH/EPDCCH transmissions within each cell. Furthermore, to support different radio channel conditions, link adaptation can be used, where the code rate of the PDCCH/EPDCCH is selected by adapting the resource usage for the PDCCH/EPDCCH to match the radio channel conditions.
Carrier Aggregation
The LTE Release 10 standard (and subsequent releases) supports bandwidths larger than 20 Megahertz (MHz). One important requirement on LTE Release 10 is to assure backward compatibility with LTE Release 8. This should also include spectrum compatibility. That would imply that an LTE Release 10 carrier that is wider than 20 MHz should appear as a number of LTE carriers to an LTE Release 8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular, for early LTE Release 10 deployments, it can be expected that there will be a smaller number of LTE Release 10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Release 10 carrier. The straightforward way to obtain this would be by means of CA. CA implies that an LTE Release 10 terminal can receive multiple CCs, where the CCs have, or at least the possibility to have, the same structure as a LTE Release 8 carrier. CA is illustrated in FIG. 4.
The number of aggregated CCs as well as the bandwidth of the individual CCs may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal. A terminal may, for example, support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
Cross-Carrier Scheduling
Scheduling of a CC is done on the PDCCH or EPDCCH via downlink assignments. Control information on the PDCCH/EPDCCH is formatted as a DCI message. In LTE Release 8, a terminal only operates with one downlink and one uplink CC and, therefore, the association between downlink assignment, uplink grants, and the corresponding downlink and uplink CCs is clear. In LTE Release 10, two modes of CA need to be distinguished. The first mode is very similar to the operation of multiple LTE Release 8 terminals. In particular, in the first mode, a downlink assignment or an uplink grant contained in a DCI message transmitted on a CC is either valid for the downlink CC itself or for an associated (either via cell-specific or UE specific linking) uplink CC. A second mode of operation augments a DCI message with the Carrier Indicator Field (CIF). A DCI containing a downlink assignment with CIF is valid for the downlink CC indicted with CIF and a DCI containing an uplink grant with CIF is valid for the indicated uplink CC. The DCI transmitted using EPDCCH, which was introduced in LTE Release 11, can also carry CIF which means that cross carrier scheduling is supported also when using EPDCCH.
WLAN
In typical deployments of a WLAN, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is used. This means that the channel is sensed, and only if the channel is declared as Idle, a transmission is initiated. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is found Idle. When the range of several Access Points (APs) using the same frequency overlap, this means that all transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP which is within range can be detected. Effectively, this means that if several APs are within range, they will have to share the channel in time, and the throughput for the individual APs may be severely degraded. A general illustration of the LBT mechanism is shown in FIG. 5.
Licensed Assisted Access (LAA) to Unlicensed Spectrum Using LTE
Up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that the LTE system does not need to care about the coexistence issue and the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited and, therefore, cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, discussions are ongoing in 3GPP to initiate a new study item on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, when using unlicensed spectrum, LTE would need to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same manner in unlicensed spectrum as in licensed spectrum can seriously degrade the performance of Wi-Fi as Wi-Fi will not transmit once it detects the channel is occupied.
Furthermore, one way to utilize the unlicensed spectrum reliably is to defer essential control signals and channels on a licensed carrier. That is, as shown in FIG. 6, a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. In the present disclosure, a SCell in an unlicensed spectrum is referred to as a License Assisted (LA) SCell.
Periodic CSI measurements can be configured in LTE, where the UE is measuring the channel on CSI-RS in predefined subframes with a periodicity T={5,10,20,40,80} ms. If the eNB detects, by using LBT, that the LA SCell channel is occupied at the configured subframe of a CSI-RS transmission, then the eNB may not be able to transmit the CSI-RS on that LA SCell. In such a subframe, the UE will not measure on a transmitted CSI-RS but on a signal transmitted by the equipment or node occupying the channel. This will lead to corrupted CSI estimates and downlink throughput degradation, which is a problem. Under rare occasions, regulations allow the eNB to transmit CSI-RS even in an occupied subframe (less than 5% duty cycle); however, this would lead to interference in the CSI estimation, which is a problem.
Thus, there is a need for systems and methods for obtaining accurate CSI estimates for a LA SCell.